Big-Block Heads Shootout - The Big O Vs. The Big R

Oval ports and rectangular port heads

Richard Holdener Jul 18, 2012 0 Comment(s)
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Two issues back, we subjected a 468-inch big-block Chevy to a variety of different oval-port cylinder heads, ranging from stock peanut-port-style heads to full CNC-ported versions (see “The Big O”). In that story, we attempted to dispel the myth that any powerful big-block combination must include rectangular-port cylinder heads. Equipped with the right oval-port heads, the 468 easily exceeded 600 hp on the engine dyno. Most of the oval-port heads tested in part 1 offered airflow that would support another 100 hp on the right combination.

Contrary to popular opinion, our combination was limiting the potential of the oval-port cylinder heads, not the other way around. It is true that all of the factory performance big-blocks sported rectangular-port heads back in the day, but much has changed since the first muscle car era. Not only does a good set of aftermarket oval-port heads outflow the factory rec-port stuff, it does so with reduced port volume. Big, lazy ports are ideally suited for neither performance nor street use, while smaller, efficient ports offer the best of both worlds. Nowhere was this more evident than in the fact that a couple of manufacturers chose to supply oval-port heads once again on this larger (and more powerful) 496 test engine.

A common stroker displacement, our 496 was the result of combining a 0.060-inch overbore with a 4.25-inch stroker crank. Our BBC rotating assembly came from the experts at Scat Enterprises. The BBC combination featured a 4340 forged steel crank combined with a set of matching 6.385-inch, I-beam rods. We chose a Scat crankshaft design specifically for our late-model, one-piece, four-bolt, Gen-6 block. The Scat crank and rods were combined with a set of forged pistons from JE. Offered as part of their SRP line up, the JE Pistons featured 18cc domes to produce a static compression ratio of 10.0:1 with typical 120cc combustion chambers.

The short-block was built not to maximize power production, but rather to demonstrate what is possible for street use in a performance driver. We wanted all of our testing to be run on pump gas, so we kept the static compression at a reasonable level. L&R Automotive was responsible for machining and balancing of the combination, while Total Seal came through with a set of performance rings to ensure proper sealing.

The displacement ensured that our 496 would be more powerful than the 468 used in part 1, but we hedged our bets with the installation of a wilder cam profile as well. Balancing the street/performance theme, we chose a small solid roller profile from Comp Cams. The 300BR-14 offered 0.652 lift, a 255/262 duration split and 114-degree LSA. To work in the Gen-6 block, the cam was teamed with a set of 0.300-tall solid roller lifters and a double roller timing chain.

Since the cam and timing chain were designed for a Gen-4 block, it was necessary to eliminate the factory cam retaining plate in favor of a traditional cam button. The swap also required the use of a custom front cover (PN217) from Comp Cams to provide the necessary room for the double-roller chain (factory Gen-6 covers must be run with a single-roller chain).

Finishing touches on the test mill included an Edelbrock Victor Jr. 454-R intake, a Holley 950 HP carb and SFI-approved, neutral damper from Procomp Electronics.

Prior to running the test engine, all of the heads were treated to flow bench testing to correlate the flow potential to the power gains. In truth, heads like the Brodix BB-3 Xtra O offered over 400 cfm--enough to support more than 800 hp. We were just scratching the surface or their potential with our 496 street/strip combination. After bench testing, we subjected the heads to port and chamber volume measurements. Since the compression ration is a function of the chamber volume, chamber size has a significant effect on the power curve (to the tune of 3-4 percent per point). Measurements indicated that our test heads varied greatly, from a low of 107cc to a high of 123cc. Bear this in mind when viewing the power numbers.

It is again worth mentioning that though this was originally to be a rec-port-only head test (to follow the oval-ports tested in part 1), some of the manufacturers supplied oval-port heads for this 496. Don't look down you noses at oval-port heads, as testing from both stories suggests that if anything, oval is the new square.




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